To accurately and faithfully transfer or project a fine pattern of a mask onto a photosensitive substrate (wafer) with an exposure apparatus for manufacturing semiconductor devices by photolithography, it is essential to extract a portion of the exposure light (radiation) in the optical path between the light source that supplies the exposure light and the photosensitive substrate, and to perform various measurements on the extracted light.
In a conventional exposure apparatus, a light-dividing member such as a half mirror is positioned in the optical path between the light source and the photosensitive substrate. The light-dividing member then extracts a portion of the exposure light, which in turn, is detected by a photoelectric detector. Various measurements are conducted on this portion of the exposure light.
The fine pattern developed on the substrate during the photolithographic step, in particular, the pattern line width, must fall within a certain permissible values. To achieve the necessary values, the accuracy of the exposure time and illumination intensity (i.e., exposure dose) must be on the order of 1% or lower. Therefore, it is essential to have a mechanism that constantly measures, during the exposure operation, the exposure time and illumination intensity of the exposure light guided to the photosensitive substrate.
One measurement of diverted exposure light is used to control the exposure dose. The following, with the aid of the schematic diagram in FIG. 1, explains prior art exposure technique for controlling the exposure dose.
FIG. 1 shows a prior art exposure apparatus EA1. In FIG. 1, an exposure light beam BI is supplied from a light source 101, such as a KrF excimer laser that emits pulsed light of a 248 nm wavelength, or an ArF excimer laser that emits pulsed light of a 193 nm wavelength. Light beam B1 is shaped to a predetermined light beam cross-sectional pattern by a beam shaping optical system 102. Light beam B1 then enters a flyeye lens 104 which serves as an optical integrator, via a folding mirror 103. Numerous light source images (secondary light sources) S2 are formed by flyeye lens 104. The light beams (not shown) from numerous light source images S2 are condensed via a lens 106 and a condenser optical system 109. Condenser optical system 109 then superimposes the light beams and uniformly illuminates a mask 110.
When mask 110 is uniformly illuminated, the pattern image of the mask is projected and transferred onto a photosensitive substrate (wafer) 113 by a projection optical system 111.
An aperture stop 105 is provided on the exit side of flyeye lens 104, and an aperture stop 112 is provided in projection optical system 111 between a lens 11a and a lens 111b. In addition, a folding mirror 108 is provided between a lens 109a and a lens 109b in condenser optical system 109.
To measure the exposure dose, exposure apparatus EA1 uses reflected light amplitude-divided by a surface 107a of a half mirror 107. Half mirror 107 is arranged between lens 106 and lens 109a in condenser optical system 109 and directs the reflected amplitude-divided portion of the light thorough condenser lens 114, which condenses the reflected light. The light then strikes a photoelectric detector 115.
The light-receiving surface of photoelectric detector 115 is arranged optically conjugate to wafer 113. The change in the illumination intensity on photoelectric detector 115 is proportional, to a high degree of accuracy, to the change in illumination intensity on wafer 113. Consequently, the exposure time and illumination intensity are controlled with high precision by feeding the measurement results from photoelectric detector 115 to an exposure dose control apparatus (not shown) and then stopping the output from light source 101, or by cutting off the supply of exposure light by a cutoff means, such as a shutter (not shown) placed in light beam B1.
Another measurement technique using exposure apparatus EA1 corrects so-called irradiation fluctuations in projection optical system 111 caused by heat accumulated in the system. The optical characteristics of projection optical system 111 changes during use due to heat accumulated by the absorption of exposure light passing through the projection optical system.
This second technique uses a second condenser lens 116 and a second photoelectric detector 117 arranged in the direction of reflection on the reverse side of half mirror 107. Some of the exposure light is reflected by wafer 113 and travels back through the system via projection optical system 111, mask 110, lens 109b, folding mirror 108 and lens 109a. A portion of this light is reflected by a surface 107b of half mirror 107 through condenser lens 116. This light is then photoelectrically detected by second photoelectric detector 117.
Thus, the amount of change in the optical characteristics of projection optical system 111 is calculated using the output from photoelectric detector 115 obtained from light reflecting from surface 107a of half mirror 107 and comparing it to the output from second photoelectric detector 117 obtained from light reflecting from surface 107b of half mirror 107. Changes in optical characteristics of projection optical system 111 are corrected by changing the movement of the lenses or the pressure in projection optical system 111.
The relationship (condition (1)):R∝λ/NA  (1) defines the resolving power of projection optical system 111 in exposure apparatus EA1. Therein, R is the pitch of the pattern at the resolution limit formed on the photosensitive substrate, λ is the exposure wavelength, and NA is the numerical aperture of projection optical system 111.
As shown by condition (1) above, the shorter the exposure wavelength, the smaller the pitch of the pattern can be imaged at the resolution limit. Thus, to form finer patterns on the photosensitive substrate, it is desirable to perform exposure with exposure light having a wavelength as short as possible.
The shortest wavelengths presently being considered for photolithographic exposure apparatus is in a region called soft X-rays, which are on the order of 5 to 20 nm.
Optical materials that transmits soft X-rays in the 5 to 20 nm wavelength range do not exist. Thus, it is impossible to assemble an optical system using lenses to form an appropriate projection exposure apparatus. To assemble an appropriate projection optical system for a projection exposure apparatus to manipulate soft x-rays, one has to use a plurality of reflective mirrors having predetermined curvatures.
To accurately and faithfully transfer a fine pattern of a mask onto a wafer, as discussed earlier, it is essential in an exposure apparatus that uses soft X-rays to extract a portion of the exposure radiation in the exposure optical path between the light source suppling the exposure light and the wafer, and to perform the necessary measurements on that extracted light. Use of a light dividing system such as a half mirror to extract one part of the exposure light in the exposure optical path is highly desirable.
For example, if a surface equivalent to the surface to be exposed (e.g., wafer 113), is formed on the light receiving surface of photoelectric detector 115 for the purpose of controlling the exposure dose, as discussed earlier, the exposure light beam must be amplitude-divided by a half mirror (e.g., half-mirror 107). Alternatively, if the exposure light beam is wavefront-divided using an optical member that wavefront-divides the exposure light-beam in place of a half mirror, a serious degradation in detection accuracy occurs. This problem arises because the part of the light beam guided to photoelectric detector 115 and the part of the light beam that reaches the surface to be exposed, are irradiated from light source luminescent spot regions at different locations, or are light beams irradiated from light source luminescent spot regions in different directions.
However, significant problems exist in fabricating a half mirror that can amplitude-divide electromagnetic radiation beam having a wavelength in the soft X-ray region.
Thus, what is needed is an optical member for soft X-rays which has the same effect as a half mirror that amplitude-divides a light beam in the visible or near visible range, an apparatus which can detect the intensity and the like of soft X-ray exposure light or electromagnetic radiation, and an apparatus that can perform various measurements for controlling the exposure process to achieve optimal exposure results.